Integrated circuits experience circuit failures from a variety of causes. For example, problems in the manufacturing process can result in defects that prevent the circuit from operating properly. Integrated circuits are tested at the manufacturing facility for proper operation before being shipped to customers. However, the integrated circuits can have defects that are not found during manufacturing test due to the inability to test every circuit node in the integrated circuit. Also the integrated circuit can operate properly at the factory but subsequently fail when placed in a larger product that is sold to an end user. The subsequent failures can be caused by circuit degradation over time, mechanical stresses leading to cracks and voids, and chemical contamination from mobile ions. When a failure occurs, whether due to the manufacturing process, design, reliability, or incorrect usage of the integrated circuit, there is a need to isolate the failure and determine the source of the failure in order to take corrective action.
Integrated circuit engineers typically determine the operation in which the failure occurs and then identify the circuit element that caused the failure. A test program can typically be used to identify the operation. However isolating the actual failing circuit is much more difficult. Historically, engineers removed passivation covering the chip and placed tiny needles, and subsequently electron beams, on exposed metal to capture signals and compare the captured signals to expected results. However, with the advent of flip-chip technology and as integrated circuit manufacturing technology progressed, circuit features became too small for mechanical probes, leading engineers to adopt laser probing.
With laser probing, also known as optic probing or electro-optic probing, a laser source is focused at a single node of an integrated circuit, and the characteristics of the reflected laser light indicate changes in the voltage of the node over time. Typical laser probing uses visible light or infrared radiation, and the chip is probed from the backside, i.e. the non-active surface. This technique has allowed probing resolution down to about 200 nanometers (nm). However as minimum transistor geometries have shrunk to much smaller sizes such as 16 nm and 14 nm, it has become difficult to discern the operation of a single transistor using laser probing, especially in the vicinity of other active transistors.
One known technique to solve these problems is to probe the integrated circuit die from the backside using shorter wavelength light, such as light in the visible spectrum despite silicon being highly absorptive in the visible spectrum. Though it achieves better resolution, this technique creates other problems. First, it requires the integrated circuit die to be thinned down to below 5 microns (μm) to overcome the losses in signal via absorption in the substrate, making it difficult to analyze failures. This process adds risk of damage caused by thinning the die, and affects the thermal dissipation in the active circuits. Second, because of the reduced wavelength, the light itself can change the behavior of the circuit. Thus this technique has proved to be inadequate.
In the following description, the use of the same reference numbers in different drawings indicates similar or identical items. Unless otherwise noted, the word “coupled” and its associated verb forms include both direct connection and indirect electrical connection by means known in the art, and unless otherwise noted any description of direct connection implies alternate embodiments using suitable forms of indirect electrical connection as well. Also various components are referred to as “optics” or “optical”, but it is to be understood that these names do not imply that the electromagnetic signals are necessarily within the visible range.